1. Field of the Invention:
This invention relates to the field of mechanical refrigeration and within that field to the utilization of air cooled condensers located outdoors and subject to high and low ambient temperatures and to controls for these air cooled condensers which allow them to work at full capacity during high temperature conditions and cause them to work at such reduced capacity during low temperature conditions as required to maintain the condensing pressure and therefore the pressure of the liquid in the liquid line supply conduit at or above a predetermined minimum.
2. Description of the Prior Art:
FIG. 1 exemplifies the present state of the prior art in condenser capacity controls. The motor-driven compressor 1 discharges compressed refrigerant vapor through discharge line 20 to air cooled condenser coil 170 in which it is condensed to a liquid by the action of the fan 176 drawing cool air over the condenser coil. The cool, condensed liquid leaves the condenser coil by way of its outlet manifold 182 and travels by way of conduit 200 to receiver 60, where it collects in a pool 70 and then is transmitted via liquid line 110 and liquid solenoid 120 to one or more evaporators 150, each under the control of an expansion valve 140.
The expansion valve modulatingly controls the flow of refrigerant liquid to the evaporator 150, feeding just enough to keep the evaporator tubes fully flooded without any liquid over-spilling into suction line 160. The refrigerant vapor resulting from the evaporation of the liquid in evaporator 150 is conveyed to the compressor 1 by way of suction line 160 for recycling.
Liquid solenoid 120 in liquid line 110 allows and prevents flow of liquid refrigerant to expansion valve 140 in accord with the requirements of a thermostat or other system control device not shown. The condenser capacity control includes bypass line 50 connecting discharge line 20 and condenser outlet line 200 and three control valves: First: discharge line regulator 35, in discharge line 20 between the condenser inlet and the point where bypass line 50 is connected. This regulator is of the type which senses inlet pressure and tends to close when the pressure drops below its predetermined setting (usually 110 PSI for refrigerant 12). It tends to open fully when the pressure rises above its predetermined setting and it tends to throttle between open and closed position at intermediate pressures. Second: Control valve 40, installed in bypass line 50, is a spring-loaded check valve whose spring load prevents it from opening until the pressure differential across it has increased to 15 or more PSI. In the alternative, control valve 40 is an outlet pressure regulating valve set to sense the pressure at the receiver and to open when that pressure falls below the predetermined valve setting, (usually 110 PSI for R-12). Third: check valve 90, installed in condenser outlet line 200 between the condenser outlet manifold and the point where bypass line 50 connects. When the ambient temperature around the air cooled condenser is about or above 75.degree. F, control valve 35 is open, control valve 40 is closed, check valve 90 is open, with the result that refrigerant vapor will freely enter the condensing coil 170 and the condensed refrigerant enter the receiver. Under this operating condition the condenser operates at essentially one hundred percent of its capacity and the head pressure which occurs is determined solely by the full condenser capacity, the load on it and the temperature of the air traversing the coil.
When the outdoor ambient drops to a temperature below approximately 75.degree. F, the pressure in discharge line 20 drops below 110 PSI, the setting of valve 35. Valve 35 therefore begins to throttle toward the closed position. When the pressure in receiver 60 drops below 110 PSI, the predetermined setting of pressure regulator valve 40, it begins to open. Now, some of the discharge vapor, which under summer conditions would have flowed directly to condenser coil 170, is able to bypass the condenser coil by way of bypass line 50 into the receiver, where it mixes with and condenses in the cool liquid refrigerant leaving the condenser coil 170, raising its temperature to about 95.degree., the temperature that corresponds to the pressure setting of valve 40. Liquid refrigerant in condenser coil 170 cannot leave the condenser until its pressure is equal to or slightly above the pressure in the receiver. Therefore, the condensed liquid is retained in condenser coil 170 until a sufficient number of its tubes have been flooded with liquid refrigerant to reduce its condensing capacity to the point where the pressure has risen slightly above the receiver pressure. Then check valve 90 pushes open and the cool refrigerant flows from condenser 170 to the receiver. While flowing, it mixes with discharge vapor bypassed through conduit 50 and control valve 40 and is warmed to the desired 95.degree. F temperature. The principle of operation of this system requires that all the liquid in the receiver by warmed, significantly reducing the refrigeration capacity of the system from the capacity it would have had if the liquid had not been warmed. All condenser capacity controls which flood the condenser must have enough extra refrigerant charged into the system initially to achieve this flooding. In the winter, the extra refrigerant resides in the condenser. In the summer this extra refrigerant is released from the condenser and resides in the receiver.
The receiver must have enough refrigerant holding capacity to store in the summer all of the refrigerant liquid required to flood the condenser coil 170 under the coldest conditions in the winter and still have some remaining space left over.